1. Field of the Invention
The present invention relates generally to the field of heat recovery systems. More specifically, the present invention relates to a heat recovery circuit that is capable of receiving turbine exhaust during all phases of turbine operation, including start up and shut down phases of operation without the use of a dump stack.
2. Description of Related Art
Reference is made to FIG. 1 that shows a typical prior art heat recovery circuit. Heat recovery circuits 50 utilize heat energy within the exhaust from a gas turbine 53 to produce super heated steam from sub-cooled water. More specifically, air from an air supply line 51 and fuel from a fuel supply line 52 are mixed and combusted within a gas turbine 53. The combustion of the air and fuel within the gas turbine 53 rotate turbine wheels within the gas turbine 53, the rotational energy of the turbine wheels is transferred to a generator 54 or other rotating machinery via a shaft 55, thereby transforming the energy of combustion within the gas turbine 53 into electrical or mechanical energy from the generator 54.
The exhaust gas exiting the gas turbine 53 primarily comprises high temperature combustion gases. During normal operation of the heat recovery system 50, the high temperature gases are directed to a heat recovery unit 67, such as a heat recovery steam generator (HRSG). As is well known, during normal operation the turbine exhaust line block valve 62 is kept open while the atmospheric exhaust line block valve 58 is kept closed. Also directed to the heat recovery unit 67 is sub-cooled water via a sub-cooled water line 66. Additionally, there is a forced, draft fan (FD) 70 which is used to supply air to duct burners (not shown) commonly employed to increase steam production over and above that generated by the heat from the turbine exhaust alone. Heat exchangers (not shown) within the heat recovery circuit 50 provide for an exchange of heat energy from the high temperature gases to the sub-cooled water within the heat recovery unit 67. The heat energy that is transferred to the sub-cooled water from the high temperature gas generally is sufficient to cause vaporization of the sub-cooled water thereby producing steam (either super heated or saturated). The steam exits the heat recovery unit 67 via the steam outlet line 68. The steam within the steam outlet line 68 can be piped to a steam turbine (not shown) and used to drive the steam turbine or can be used as process heat. As is the case with the gas turbine 53, the steam turbine can be coupled to a generator so that operation of the steam turbine results in the production of electrical energy.
During normal operation of the gas turbine 53 the pressure of the gas exiting the gas turbine sufficiently exceeds the pressure within the heat recovery circuit 50 so that the turbine exhaust gas readily flows through the heat recovery circuit 50. While the FD-fan is operating, however, during the start up and shut down phases of the gas turbine 53, the pressure within the heat recovery circuit 50 can exceed the pressure of the gas exiting the gas turbine 53 thereby producing a backpressure where the gas turbine 53 connects to the heat recovery circuit 50. This backpressure can have a deleterious effect on the performance of the gas turbine 53, and in some cases even damage the gas turbine 53. To eliminate backpressure during start up and shutdown, the general practice is to redirect the gas exiting the gas turbine 53 from the heat recovery unit 67 into the atmosphere via a dump stack 56. Typically this is accomplished by having the block valve 58 in dump stack 56 open while the turbine exhaust line block valve 62 is closed.
Directing the exhaust gases to the atmosphere can overcome backpressure problems inherent in a heat recovery circuit 50. However, directly emitting combustion exhaust gases into the atmosphere presents other issues, such as environmental concerns. These exhaust gases often comprise nitrogen based oxides (NOx), sulfur-based oxides (SOx), carbon monoxide (CO), or combinations thereof—that are considered pollutants and are thus closely monitored by state and federal environmental agencies. Although the exhaust gases are ultimately released from the heat recovery device 67 to the atmosphere via a stack 11, a typical heat recovery device 67 includes means for reducing the pollutants. These means can include chemical injections, selected catalytic reduction, or other pollutant reduction techniques. Unfortunately, releasing these exhaust gases into the atmosphere at or close to the exit of the gas turbine 53 bypasses the pollution treatment step that occurs in the heat recovery device 67 and therefore results in a release of untreated gases. Since such releases have been determined to be environmentally detrimental, governmental agencies that monitor such heat recovery circuits 50 often assess fines when these releases occur. Therefore, a need exists for a heat recovery circuit that is capable of receiving exhaust combustion gases during all stages of a gas turbine's operation. Further a need exists for a heat recovery circuit that eliminates backpressure occurrences within the heat recovery circuit without emitting untreated exhaust gases into the atmosphere.